A modern battery far outlives the rest of the vehicle when you look at the charging cycles. So you can afford to use some of it for home electricity storage. AFAIK some cells types easily last 3,000 cycles at >80% capacity. Assume 400km/charge you’d be looking at 1.2mil kilometers of driving just based on the battery.

Also, there is a a natural limit to how much you’re using, considering you also want to use your vehicle for driving as well.

Not every charge/discharge wears on the battery the same. Staying at 0% or 100% is worse than staying around the middle. Charging and discharging extremely quickly wears more than slow charges/discharges. And high temperatures are bad for battery health in general.

Electric car batteries are tuned much more conservatively than phone batteries and are massively over-provisioned because US consumers are nervous about exactly the thing you're talking about. That's not too surprising because to a large degree the car IS the battery, but battery wear is generally very well mitigated (with the exception of the Nissan Leaf).

Early Nissan Leafs. At this point, the newer used ones include battery health management, and should be similar to other used EVs.

Note that getting an old one with a dead battery makes even less sense than you'd guess: It'll burn out any replacement battery just as fast, assuming you can actually get one.

If I was able to use my EV as an external battery for my home, I could potentially save ~$100 per month on my power bill, and that would represent <1/2 additional charge cycle per day. I'm pretty sure that would be worth it. This is going to depend on the power rates in ones particular area though.